WO90/05969 discloses a traffic congestion monitoring system comprising infra-red monitoring units bolted to the sides of bridges over a motorway network and emitting information as to traffic congestion at their locations, a control centre which receives and transmits the information, and paging units in respective vehicles and receiving the information and visually displaying the same upon diagrams of the network or zones thereof.
The infra-red monitoring units in effect look at one location underneath a bridge or gantry to measure the average speed. Of course this works well on motorways and dual carriageway standard trunk roads where there is a continual flow of traffic. However, unless these units are put very close together (300-500 meters), they are an inaccurate way of measuring average speed or journey time on roads where there are traffic flow interruptions caused by traffic lights, roundabouts and other obstacles.
Vehicle licence-plate reading systems are known but these have been designed for law enforcement purposes, tolling and entry control for car parks, for example. These systems require a very high degree of accuracy as clearly if a licence-plate is misread it may not allow access through a barrier, or if used for enforcement cannot be legally enforceable. Because of this general requirement for a very high degree of accuracy, hardware platforms for reading licence-plates are very expensive and the data content and thus cost that would be required to communicate these licence-plates over a radio link would be very high because of the detail involved.